1. Field of the Invention
The present invention relates to a baseball or softball bat and, more specifically, to a baseball or softball bat having superior restitution characteristic.
2. Description of the Background Art
Baseball or softball bats formed of various materials including wood, metal such as titanium, titanium alloy and aluminum alloy, and fiber-reinforced plastic prepared by impregnating carbon fiber, glass fiber or the like with matrix resin and curing, have been available in the market.
Conventionally, in order to improve restitution characteristic of a bat, Japanese Patent Laying-Open No. 2002-052108 (Patent Document 1) discloses a bat having a tubular insert arranged on the inner circumferential side of a hitting portion and an elastomer filled between an inner wall and an outer circumferential surface of the tubular insert at the hitting portion. For further improvement of restitution characteristic, Japanese Patent Laying-Open No. 2001-190724 (Patent Document 2), for example, discloses a bat having fiber-reinforced plastic laminated on an outer circumference of a metal pipe, in which weak boundary layer is provided between the metal pipe and the fiber-reinforced plastic layer. Further, International Patent Publication No. 00/23151 (Patent Document 3) discloses a bat having a metal tubular member embedded in a tubular base forming the hitting portion of the bat, with a weak boundary layer formed on the surface of the metal tubular member. Here, the weak boundary layer is formed of a release agent such as wax or a release film such as polyethylene film, so as to prevent adhesion of members facing with each other with the weak boundary layer interposed.
The conventionally proposed bats described above, however, have the following problems. Specifically, in the bat disclosed in Patent Document 1, though different materials are laminated at the hitting portion of the bat, these layers are adhered and integrated to each other, and therefore, restitution characteristic is not much improved at the hitting portion of the bat at the time of hitting the ball. Further, dependent on the adhesion strength between the tubular insert and the inner wall of the hitting portion, the tubular insert may possibly be separated from the inner wall at the hitting portion, due to repeated hitting during use.
The bat disclosed in Patent Document 2 is said to improve restitution characteristic at the hitting portion of the bat, as the fiber-reinforced plastic layer and the metal pipe can deform independently when hitting the ball. In order to fix the position of fiber-reinforced plastic with respect to the metal pipe, however, the laminated body and the metal pipe are adhered without forming the weak boundary layer, at end portions (opposite ends in the axial direction of the bat) of the laminated body of fiber-reinforced plastic. Therefore, the laminated body does not move along the axial direction of the bat. Further, in the bat disclosed in Patent Document 3 also, the metal tubular member is embedded in the tubular base and, therefore, the metal tubular member does not move along the axial direction of the bat.
Therefore, in the bats disclosed in Patent Documents 2 and 3 above, vibration and shock of the bat at the time of hitting the ball would be alleviated, possibly at a high percentage, by energy loss due to the radial motion of the laminated body and the metal pipe or of the metal tubular member and the tubular base. As a result, such vibration and shock can be alleviated only moderately, and hence, feeling of hitting with the bat could be unsatisfactory.